[clang] [C++20][Modules] Load function body from the module that gives canonical decl (PR #111992)

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

The missing bit was `-O1`, as soon as I've added that, I am now getting the error with this commit and not getting any error without this commit.
Here is a proper reproducer: [friends.tgz](https://github.com/user-attachments/files/17950471/friends.tgz)

Please take a look and let me know if this helps get to the bottom of the problem.
As discussed, it might very well be some unrelated problem getting surfaced (e.g. due to different deserialization order), but it'd be nice to get some hints on what's exactly happening there to make progress on that.



https://github.com/llvm/llvm-project/pull/111992